The invention relates to a system for locally testing a modem employing frequency shift keying and being coupled to a two-wire transmission line, the two frequencies transmitted by the modem being lower than the two received frequencies and being produced by a modulator in the form of a multivibrator controlled by the data and connected to a transmission filter, the test system comprising means for looping the modem at the transmission line side in response to a test control signal.
A modem of the type considered above is, for example, a modem as defined in the CCITT recommendation V 23, which transmits data at a rate of 75 Baud in the form of two frequencies f.sub.0 .+-..DELTA.f, where f.sub.0 =420 Hz and .DELTA.f=30 Hz, and receives data at a rate of 1200 Baud in the form of two frequencies F.sub.0 .+-..DELTA.F, where F.sub.0 =1700 Hz and .DELTA.F=400 Hz. These modems may find a very large range of usage in applications such as electronic telephone directories, video text, etc. and it is important that they are simple and inexpensive to implement. A solution to realize the modulator is the use of a multivibrator circuit whose frequency is controlled by the data, for example as disclosed in U.S. Pat. No. 4,039,952.
Moreover, a local test system must be provided in these modems, this test being performed after a looping of the modem at the transmission line side, which is designated looping 3 in the CCITT recommendation V 54. According to this recommendation, the looping realized for the test must include the maximum number of circuits used during normal operation.